共查询到20条相似文献，搜索用时 31 毫秒
1.
Charles R. Wilson 《Geophysical Journal International》1971,26(1-4):179-181
Summary Observations at Inuvik (70.4° dipole latitude) have shown that supersonic motions of auroral arcs that sweep across the zenith from south to north during poleward expansions of auroral substorms do not generate observable auroral infrasonic waves. This is in contrast to the fact that equator-ward supersonic motions of similar auroral arcs do produce large amplitude infrasonic bow waves. These results imply an asymmetry in the basic generation mechanism of infrasound within the auroral electrojet arcs. 相似文献
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Summary. The paper reports studies of the three-dimensional magnetospheric—ionospheric current systems which produced polar magnetic substorms on 1974 September 7 and September 18. The data were magnetic perturbation fields observed with a two-dimensional array of 23 three-component magnetometers located in western Canada beneath the auroral oval. In an earlier study of a substorm of September 11 (Bannister & Gough) the fields fitted calculated field for a Boström Type 1 current loop with field-aligned currents at east and west ends of the ionospheric segment, and with uniform current density across the width. The substorms here reported could not be modelled with uniform current density. An inverse method due to Oldenburg was therefore used to estimate current density distributions, and satisfactory fits of calculated to observed field resulted. Each substorm was modelled at six representative epochs. In general the principal ionospheric current seem by the array was westward. At four epochs of the September 7 substorm and throughout the September 18 substorm, significant eastward ionospheric current (or its equivalent in terms of the fields produced) was observed north of the westward electrojet. Northwestward bends in the ionospheric current segments were found at four epochs on September 7 and at three epochs on September 18. As in the September 11 substorm (Paper 1), these bends were either west of or close to magnetic midnight. In some cases the bends may follow the auroral oval, but in others they are sharper and may be associated with the Harang discontinuity. East of geomagnetic the ionospheric currents tend to run in a constant geomagnetic midnight latitude range. The developments of the three substorms, of September 7, 11 (Paper 1) and 18, are compared. They showed a variety of shifts in longitude, though all moved eastward relative to magnetic midnight. 相似文献
4.
The influence of barometric-pressure variations on gravity 总被引：2，自引：0，他引：2
Summary. The superconducting gravimeter has been used to measure the influence of barometric pressure on gravity in the frequency range 0.1–10 cycles/day. These measurements show that the incoherent barometric fluctuations are the major cause of random fluctuations in local gravity and account for much of the 'noise' on our gravimeter records. A simple model has been constructed which adequately explains the response of gravity to the local pressure fluctuations. These measurements also show a response to the global atmospheric tides at S 1 and S 2 which is much larger than the response to local fluctuations. Although this behaviour is most likely due to the response of the world-wide oceans to the atmospheric tides, no theoretical model has yet been constructed. 相似文献
5.
Summary During the past 10 years the Geoacoustics Group of NOAA's Wave Propagation Laboratory studied travelling low-frequency pressure variations related to thunderstorms and severe weather. Two general categories of waves were associated with severe weather conditions: 'subsonic' pressure disturbances and infrasonic waves with acoustic velocities. The low-frequency pressure variations were measured at the Earth's surface using microphone arrays located at times thousands of kilometres from the severe-weather disturbance. The radiated infra- sound was related to thunderstorms penetrating the tropopause and spectral analyses were performed on several signals. Possible practical applications to storm warning and classification are discussed for both infrasound and 'subsonic' pressure disturbances. Past measurements of these signals are reviewed. 相似文献
6.
Coseismic well-level changes due to the 1992 Roermond earthquake compared to static deformation of half-space solutions 总被引：3，自引：0，他引：3
The M w 5.4 Roermond earthquake of 1992 April 13 was one of the strongest events during the last 500 years in Central Europe. For the period March–May 1992, we collected records of 194 continuously operating well-level sensors, mostly located within 120 km of the epicentre. Nearly all wells penetrate unconfined or poorly confined Quaternary deposits with high hydraulic conductivities. 81 out of 194 raw data sets show a significant dynamic or step-like response of centimetre amplitude to the passage of seismic waves. Precursory anomalies are not obvious in these records. Coseismic well-level fluctuations could reflect a redistribution of stress and pore pressure in the brittle crust. Systematic analyses of such fluctuations may improve our knowledge of the role of pore fluids in crustal rheology and earthquake mechanics. The rather high number of individual observational records for a single event allows a regional correlation of the signs and amplitudes of the coseismic steps to changes in volume strain caused by the earthquake. The coseismic strain field at the surface was calculated for a homogeneous and a layered half-space. The results show reasonable agreement in the sign of the well-level steps but the amplitudes predicted from the wells' volumetric strain responses are much smaller than those that were recorded. Clearly, the coseismic well-level steps cannot be explained by volume strain changes, as derived from linear elastic models. 相似文献
7.
Using the ground observation data at Zhongshan Station of Antarctica during July 13 to 17, 2000, the intense absorption events associated with the activities of the solar active region R9077 are analyzed. It was shown that an intense polar cap absorption event lasted more than 3 days, which was caused by the solar proton event associated with the X5/3B major flare at 1024 UT on July 13. The polar cap event started at about 1040 UT on July 14, and lasted to about 1940 UT on July 17, with a typical day night variation. At the same time, the intense solar activities extremely disturbed the magnetosphere, therefore aurora substorms occurred frequently. The energetic particle precipitation from the magnetosphere caused several absorption spikes superposing on the background of polar cap absorption. One distinct event is the absorption enhancement that started at about 0300 UT on July 15, reached its peak of 26 dB at about 0645 UT and recovered at about 1110 UT on the same day, which was the strongest absorption event observed at Zhongshan Station since the imaging riometer installed in February, 1997. Another outstanding absorption spike with pulsation occurred at about 1753 UT on 14th, its peak reached to 6 dB. 相似文献
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A new technique of eigen mode analysis, Method of Natural Orthogonal Components (MNOC) is used to analyze the ionospheric equivalent current systems obtained on the basis of magnetic data at six meridian magnetometer chains in the northern hemisphere during March 17 19, 1978. The results show that the whole current pattern for any given instant consists of a few eigen modes with different intensities. The first eigen mode exhibits a two cell current construction, characterizing the large scale magnetospheric convection and directly driven process, while the second eigen mode shows a concentrated westward electrojet at midnight sector, characterizing the substorm current wedge and the loading unloading process. The first mode consistently exists whenever during quiet periods or at substorms, and its intensity increases from the beginning of the growth phase of substorms, then quickly intensifies in the expansion phase, followed by a gradual decrease in the recovery phase. On the other hand, the intensity of the second mode remains to be near zero during both quiet time and the growth phase of substorms. Its rapid enhancement occurs in the expansion phase. These characteristics in the current patterns and the intensity variations coincide with the defined physical processes of the directly driven and loading unloading components. 相似文献
9.
A comprehensive model current system for high-latitude magnetic activity -II. The substorm component
Summary. In paper I of this series (Hughes & Rostoker) we have presented a composite model three-dimensional current system whose magnetic perturbation pattern is in good agreement with that observed at high latitudes during periods of moderate magnetospheric activity. In this paper we extend the model current system to demonstrate its ability to reproduce the magnetic perturbation pattern associated with the magnetospheric substorms. In agreement with previous workers, we find it necessary to include a region of intense equatorward current flow at the western edge of the westward travelling surge. For the particular substorm event presented in this paper, we find that the main eastward and westward electrojets intensified slightly before the appearance of the surge form, and we are able to evaluate quantitatively the amount of change involved. 相似文献
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Avery L. C. Shinneman Danuta M. Bennett Sherilyn C. Fritz Jens Schmieder Daniel R. Engstrom Aris Efting John Holz 《Journal of Paleolimnology》2010,44(2):443-464
The Nebraska Sand Hills are a distinctive eco-region in the semi-arid Great Plains of the western United States. The water
table underlying the Sand Hills is part of the High Plains/Ogallala aquifer, an important water resource for the central Great
Plains. Lake levels are affected directly by fluctuations in the water table, which is recharged primarily by local precipitation
and responds quickly to climatically induced changes in regional water balance. Instrumental records are available for only
50–100 years, and paleolimnological data provide important insights into the extremes and variability in moisture balance
over longer time scales. A set of 69 lakes from across Nebraska was used to establish a statistical relationship between diatom
community composition and water depth. This relationship was then used to develop a diatom-based inference model for water
depth using weighted averaging regression and calibration techniques. Development of the inference model was complicated by
strong intra-seasonal variability in water depth and the linkages between depth and other limnologic characteristics, including
alkalinity, water clarity and nutrient concentrations. Analysis of historical diatom communities from eight lakes allowed
for the reconstruction of lake-level fluctuations over the past several thousand years. Comparisons of the more recent portion
of these reconstructions with the instrumental Palmer Drought Severity Index (PDSI) showed that sediment records may not faithfully
reflect short-term fluctuations in water level, except where sedimentation rates are very high. However, large and persistent
changes in moisture availability were discernible even in longer, low-resolution records. Thus, diatoms are a useful addition
to the tools available for understanding past drought in the central Great Plains, especially when trajectories of change
are constrained by data from multiple sites or other proxies. 相似文献
12.
Mats G. Eriksson Raymonde Bonnefille Forename Surname Séverine Lafon 《Journal of Paleolimnology》1999,22(4):457-473
Variations in pollen assemblages and in physical and chemical composition of a dated sediment record from the small Lake Haubi in north central Tanzania, reveal lake level fluctuations since the late 19th century. Lake Haubi changed from a seasonally inundated swamp to a lake in the beginning of the 20th century. With the exception of 1942-44, when it dried out completely, it remained water filled until 1994 when it again turned into a swamp. The lake level fluctuations in Lake Haubi are largely in phase with fluctuations of the larger East African lakes levels during the 20th century, and are therefore interpreted as being mainly controlled by regional climatic fluctuations. However, the initial formation of Lake Haubi at the turn of the century was likely due to local catchment specific causes, e.g. changes in land use, as the rapid increase in the water level at this time does not correspond to other lake level records from the region. 相似文献
13.
Robert Kusi Antony White Graham Heinson & Peter Milligan 《Geophysical Journal International》1998,132(3):687-700
Magnetic field fluctuations have been recorded by an array of portable three-component magnetometers at 60 sites across the Eyre Peninsula in South Australia between December 1993 and March 1995. An additional 54 magnetometer data records, collected prior to 1989 and described by Milligan (1989) and Milligan, White & Chamalaun (1989), were included in the analysis. A major conductive feature in the crust, first noted by White & Milligan (1984) as the Eyre Peninsula Anomaly (EPA), is re-examined to assess its continuity to the north of the original arrays and to investigate its relationship with major tectonic features.
Magnetic-field time-series were converted to induction arrows in the frequency domain. These induction arrows were initially inverted using the minimum-structure 2-D Occam approach to estimate the electrical conductance of the crust. Following this, thin-sheet forward modelling was used to examine the relationship between the conductance and the dominant tectonic features. The principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km before terminating, and the conductance is in the range 3000 to 10 000 S, which decreases inland.
A strong correlation exists between the electrical conductance of the Eyre Peninsula and Bouguer gravity anomalies, and in particular the EPA is coincident with a significant Bouguer gravity gradient. There is also good agreement between the locations of the foci of earthquakes of magnitude greater than 4.0 and the EPA. We believe that the anomaly is associated with a geological fracture in the Precambrian upper crust as a result of crustal extension prior to the rifting of Australia from Antarctica in the Jurassic (160 Ma). 相似文献
Magnetic-field time-series were converted to induction arrows in the frequency domain. These induction arrows were initially inverted using the minimum-structure 2-D Occam approach to estimate the electrical conductance of the crust. Following this, thin-sheet forward modelling was used to examine the relationship between the conductance and the dominant tectonic features. The principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km before terminating, and the conductance is in the range 3000 to 10 000 S, which decreases inland.
A strong correlation exists between the electrical conductance of the Eyre Peninsula and Bouguer gravity anomalies, and in particular the EPA is coincident with a significant Bouguer gravity gradient. There is also good agreement between the locations of the foci of earthquakes of magnitude greater than 4.0 and the EPA. We believe that the anomaly is associated with a geological fracture in the Precambrian upper crust as a result of crustal extension prior to the rifting of Australia from Antarctica in the Jurassic (160 Ma). 相似文献
14.
Richard K. Cook 《Geophysical Journal International》1971,26(1-4):191-198
Summary Seismic waves caused by earthquakes radiate infrasound into the atmosphere as they proceed over the Earth's surface. Several instances of such sound waves radiated locally by seismic waves passing through the Washington, D.C., area have been observed at the infrasonic station there. A notable instance was the great Montana earthquake of 1959 August 18. Measurements of the radiated infrasound gave data on the seismic waves, including their travel times, local speeds, directions of travel, amplitudes, and waveforms. 相似文献
15.
Christopher Sanchez Evelyn E. Gaiser Colin J. Saunders Anna H. Wachnicka Nicholas Oehm Christopher Craft 《Journal of Paleolimnology》2013,49(1):45-66
Successfully rehabilitating drained wetlands through hydrologic restoration is dependent on defining restoration targets, a process that is informed by pre-drainage conditions, as well as understanding linkages between hydrology and ecosystem structure. Paleoecological records can inform restoration goals by revealing long-term patterns of change, but are dependent on preservation of biomarkers that provide meaningful interpretations of environmental change. In the Florida Everglades, paleohydrological hind-casting could improve restoration forecasting, but frequent drying of marsh soils leads to poor preservation of many biomarkers. To determine the effectiveness of employing siliceous subfossils in paleohydrological reconstructions, we examined diatoms, plant and sponge silico-sclerids from three soil cores in the central Everglades marshes. Subfossil quality varied among cores, but the abundance of recognizable specimens was sufficient to infer 1,000–3,000 years of hydrologic change at decadal to centennial resolution. Phytolith morphotypes were linked to key marsh plant species to indirectly measure fluctuations in water depth. A modern dataset was used to derive diatom-based inferences of water depth and hydroperiod (R2 = 0.63, 0.47; RMSE = 14 cm, 120 days, respectively). Changes in subfossil quality and abundances at centennial time-scales were associated with mid-Holocene climate events including the Little Ice Age and Medieval Warm Period, while decadal-scale fluctuations in assemblage structure during the twentieth century suggested co-regulation of hydrology by cyclical climate drivers (particularly the Atlantic Multidecadal Oscillation) and water management changes. The successful reconstructions based on siliceous subfossils shown here at a coarse temporal scale (i.e., decadal to centennial) advocate for their application in more highly resolved (i.e., subdecadal) records, which should improve the ability of water managers to target the quantity and variability of water flows appropriate for hydrologic restoration. 相似文献
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Summary. Sea-level records at nine ports along the coasts of Namibia and South Africa are used to establish the existence of coastal trapped disturbances in sea-level as a response to the passage of synoptic weather systems. Using spectral analysis the characteristics and spatial variability of the sea-level fluctuations are identified. The results of cross-correlation analyses performed on sea-level data at adjacent ports for two periods during 1982 are discussed in detail to examine the propagation of coastal trapped waves round the coast. 相似文献
17.
BY FIDEL A. ROIG DAVID BARRIOPEDRO RICARDO GARCÍA HERRERA DANIEL PATÓN DOMINGUEZ SUSANA MONGE 《Geografiska Annaler: Series A, Physical Geography》2009,91(2):141-157
Exactly dated ring-width chronologies derived from Pyrenean oak and sweet chestnut trees growing in northern Extremadura, Spain, were evaluated for their potential as proxies for regional precipitation and North Atlantic Oscillation variability. The relationships among tree-rings, instrumental climatic records, and three versions of the NAO index were computed for different time subperiods over the last century. The results indicate that tree-ring records reflect, with variable intensity, both short-term and long-term variations in climate. Multiple correlation and regression analysis revealed that summer precipitation appeared to be the major factor affecting tree growth at inter-annual timescales. Moreover, since fluctuations in accumulated variability in annual rainfall over southwest Iberia are controlled by winter precipitation, the accumulated rainfall (August of the year n -1 to July of year n ) and winter NAO indices are also strongly correlated with tree-ring records at interdecadal timescales. This relationship appears to be especially strong during the second half of the 20th century, which is consistent with an increase of the NAO signal in the annual precipitation during the later part of the century. These results indicate that tree-rings from western Iberia are potential proxies of the NAO variability, useful to be included in palaeoclimatic model studies. 相似文献
18.
利用IMAGE卫星FUV极光图像数据,对北半球极光椭圆带2760个亚暴事件进行统计分析,得到亚暴等待时间的统计分布,并利用简单的数学模型对卫星轨道的影响进行了讨论,得到以下结论:(1)亚暴等待时间在2-3h附近有一个峰值,较低纬度( MLat＜ 65°)发生的亚暴事件,其等待时间略长于较高纬度(MLat≥65°)发生的... 相似文献
19.
P. A. Camfield 《Geophysical Journal International》1981,65(3):553-570
Summary. An array of 26 magnetometers deployed in a tectonically active area of Quebec has mapped a boundary in terrestrial electrical conductivity for 200 km along strike. The contrast in conductivity across the boundary, from previous magnetotelluric soundings, is about one order of magnitude. Anomalous variation fields associated with electric currents flowing along the boundary are readily detected at pulsation periods only when the horizontal field is polarized transverse to the structure (the E -polarization case). The anomaly is hardly visible in transfer functions from substorms, for a number of reasons: a predominant H -polarization orientation of the substorm fields, the single order-of-magnitude contrast in conductivity, and the probably small depth extent of the structure. Attempts were made to estimate the response of a one-dimensional earth via the inductive scale length with gradients evaluated from polynomial surfaces fitted to the smoothly varying substorm fields. The results were inconsistent, owing to vertical fields with strong external components and to horizontal fields with scale lengths too small relative to their penetration distances. 相似文献
20.
Eric S. Posmentier 《Geophysical Journal International》1971,26(1-4):173-177
Summary Infrasound with frequencies of 1–16 Hz, detected by an array of four thermistor flow-meter microphones in Sterling Forest, New York, was observed to have a continuous background with peak energy distributed near 16 Hz in frequency, with amplitudes of about 1 dyn cm-2 , and arriving from the south-west and south-east at slightly above the speed of sound in air at ground level. The same array of microphones detected 5 dyn cm-2 signals from the Apollo 14. The earlier part of the 10-min signal arrived from the first stage re-entry, the later from the launch site vicinity. It is shown that aircraft beyond the visible and audible range can be detected and tracked by monitoring the infrasound emitted throughout most of the 1–16 Hz frequency band. 相似文献